Data storage device

ABSTRACT

The present disclosure relates to a data storage device, comprising a plurality of electron emitters adapted to emit electron beams, the electron emitters each having a planar emission surface, and a storage medium in proximity to the electron emitter, the storage medium having a plurality of storage areas that are capable of at least two distinct states that represent data, the state of the storage areas being changeable in response to bombardment by electron beams emitted by the electron emitters.

FIELD OF THE INVENTION

The present disclosure relates to a data storage device. Moreparticularly, the disclosure relates to a data storage deviceincorporating ballistic or quasi-ballistic electron emitters.

BACKGROUND OF THE INVENTION

Researchers have continually attempted to increase the storage densityand reduce the cost of data storage devices such as magnetichard-drives, optical drives, and dynamic random access memory (DRAM). Ithas, however, become increasingly difficult to increase storage densitydue to fundamental limits such as the superparamagnetic limit, belowwhich magnetic bits are unstable at room temperature.

Several approaches have been used to increase storage density of storagedevices. One approach is based on scanned probe microscopy (SPM)technology. In such an approach, a probe is positioned extremely closeto a storage medium. An example is atomic force microscopy (AFM) inwhich a probe is placed into physical contact with the storage medium.Another example is scanning tunneling microscopy (STM) in which theprobe is placed within a few nanometers from the storage medium toensure that the probe is within a tunneling range of the medium.Although limited success has been achieved through these approaches, itis difficult to inexpensively build a storage device having probes thatcontact or are in close proximity to the storage medium withouteventually damaging the probe and/or the surface of the medium.Moreover, in STM, the spacing must be precisely controlled. As known bypersons having ordinary skill in the art, such control is difficult toachieve.

In view of the difficulties associated with SPM, other researchers havedeveloped methods that eliminate the need for extremely close proximity.One such technique is based on near-field scanning optical microscopy(NSOM). Although avoiding the proximity problem, this technique haslimited lateral resolution and bandwidth and therefore is of limitedapplicability. Other techniques have been developed based on non-contactSFM, but these techniques typically suffer from poor resolution and poorsignal to noise ratio.

Even where increased storage density can be achieved, hurdles toeffective implementation exist. Once such hurdle is the time required toaccess data stored on the storage device the information. Specifically,the utility of the storage device is limited if a long time is requiredto retrieve the stored data. Therefore, in addition to high storagedensity, there must be a way to quickly access the data.

Recently, semiconductor-based electron sources have been developed thatcan be used in storage devices and which may avoid the difficultiesnoted above. An example of such a data storage device is described inU.S. Pat. No. 5,557,596. The device described in that patent includesmultiple electron sources having electron emission surfaces that face astorage medium. During write operations, the electron sources bombardthe storage medium with relatively high intensity electron beams. Duringread operations, the electron sources bombard the storage medium withrelatively low intensity electron beams. Such a device providesadvantageous results. For instance, the size of storage bits in suchdevices may be reduced by decreasing the electron beam diameter, therebyincreasing storage density and capacity and decreasing storage cost.

One type of electron source described in the U.S. Pat. No. 5,557,596 isthe “Spindt” emitter. As described in the patent, such an emitter has acone shape that ends in a tip from which electron beams can be emitted.Typically, the tip is made as sharp as possible to reduce operatingvoltage and achieve a highly focused electron beam diameter.Unfortunately, utilization of Spindt emitters creates other problems.First, the fabrication of sharp emitter tips is difficult and expensive.In addition, focusing the electron beam from a Spindt tip in atemporally and spatially stable manner is difficult. Furthermore, theelectron optics that provide the focusing can become complicated.Moreover, Spindt emitters do not operate well in poor vacuums. Theseproblems become especially prominent as the electron beam diameter isreduced below 100 nanometers.

From the foregoing, it can be appreciated that it would be desirable tohave a data storage device that employs electron emitters but thatavoids one or more of the problems identified above.

SUMMARY OF THE INVENTION

The present disclosure relates to a data storage device, comprising aplurality of electron emitters adapted to emit electron beams, theelectron emitters each having a planar emission surface, and a storagemedium in proximity to the electron emitter, the storage medium having aplurality of storage areas that are capable of at least two distinctstates that represent data, the state of the storage areas beingchangeable in response to bombardment by an electron beams emitted bythe electron emitters, wherein data is written to the device by changingthe state of the storage areas and data is read by the device byobserving phenomena relevant to the storage areas.

In addition, the disclosure relates to a method for storing data,comprising the steps of emitting an electron beam from an electronemitter including a planar emission surface, directing the electron beamtoward a storage medium comprising a plurality of storage areas, andbombarding one of the storage areas with electrons with the electronbeam so as to change the state of a storage area. Typically, althoughnot necessarily, the method further comprises the step of bombarding oneof the storage areas with electrons with a lower current electron beamand observing its effect on the storage area.

The features and advantages of the invention will become apparent uponreading the following specification, when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings. The components in the drawings are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesof the present invention.

FIG. 1 is a schematic side view of an example data storage device.

FIG. 2 is a schematic cross-sectional view of the data storage device ofFIG. 1 taken along line 2—2.

FIG. 3 is a schematic cross-sectional perspective view of the datastorage device of FIGS. 1 and 2 taken along line 3—3.

FIG. 4 is a partial schematic view of a storage medium of the datastorage device shown in FIGS. 1-3.

FIG. 5 is a schematic side view of a first example reading arrangementfor the data storage device of FIGS. 1-4.

FIG. 6 is a schematic side view of a second example reading arrangementfor the data storage device of FIGS. 1-4.

FIG. 7 is a schematic side view of a first electron emitter suitable foruse with the data storage device of FIGS. 1-4.

FIG. 8 is a detail view of a conductive layer of the first electronemitter shown in FIG. 7.

FIG. 9 is a schematic side view of a second electron emitter suitablefor use with the data storage device of FIGS. 1-4.

FIG. 10 is a schematic side view of a third electron emitter suitablefor use with the data storage device of FIGS. 1-4.

FIG. 11 is a schematic side view of a fourth electron emitter suitablefor use with the data storage device of FIGS. 1-4.

FIG. 12 is a schematic side view of a fifth electron emitter suitablefor use with the data storage device of FIGS. 1-4.

DETAILED DESCRIPTION

Referring now in more detail to the drawings, in which like numeralsindicate corresponding parts throughout the several views, FIGS. 1-3illustrate an example data storage device 100. It is noted that thisdevice 100 is similar in construction to that described in U.S. Pat. No.5,557,596, which is hereby incorporated by reference into the presentdisclosure.

As indicated in FIGS. 1-3 the data storage device 100 generally includesan outer casing 102 that forms an interior space 104 therein. By way ofexample, the casing 102 can include a plurality of walls 106 that definethe interior space 104. Typically, the walls 106 of the casing 102 aresealed to each other such that a vacuum can be maintained within theinterior space 104. By way of example, the casing 102 maintains a vacuumof at least approximately 10⁻³ torr within the interior space 104.Although a particular configuration is shown for the casing 102, it isto be understood that the casing can take many different forms thatwould be readily apparent to persons having ordinary skill in the art.

Within the interior space 104 is a plurality of electron emitters 108that face a storage medium 110. As described in relation to FIG. 4, thestorage medium 110 comprises a plurality of storage areas (not visiblein FIGS. 1-3). In a preferred embodiment, each storage area of thestorage medium 110 is responsible for storing one or more bits of data.

The electron emitters 108 are configured to emit electron beam currentstoward the storage areas of the storage medium 110 when a predeterminedpotential difference is applied to the electron emitters. Depending uponthe distance between the emitters 108 and the storage medium 110, thetype of emitters, and the spot size (i.e., bit size) required, electronoptics may be useful in focusing the electron beams. An example of suchoptics is provided below (FIG. 9). Voltage is also applied to thestorage medium 110 to either accelerate or decelerate the emittedelectrons and/or to aid in focusing the emitted electrons.

Each electron emitter 108 can serve many different storage areas towrite data to and read data from the storage medium 110. To facilitatealignment between each electron emitter 108 and an associated storagearea, the electron emitters and storage medium can be moved relative toeach other in the X and Y directions noted in FIG. 2. To provide forthis relative movement, the data storage device 100 can include amicromover 112 that scans the storage medium 110 with respect to theelectron emitters 108. As indicated in FIGS. 1 and 3, the micromover 112can include a rotor 114 connected to the storage medium 110, a stator116 that faces the rotor, and one or more springs 118 that arepositioned to the sides of the storage medium. As is known in the art,displacement of the rotor 114, and thereby the storage medium 110, canbe effected by the application of appropriate potentials to electrodes117 of the stator 116 so as to create a field that displaces the rotor114 in a desired manner.

When the micromover 112 is displaced in this manner, the micromoverscans the storage medium 110 to different locations within the X-Y planesuch that each emitter 108 is positioned above a particular storagearea. A preferred micromover 112 preferably has sufficient range andresolution to position the storage areas 110 under the electron emitters108 with high accuracy. By way of example, the micromover 112 can befabricated through semiconductor microfabrication processes. Althoughrelative movement between the electron emitters 108 and the storagemedium 110 has been described as being accomplished through displacementof the storage medium, it will be understood that such relative movementcan alternatively be obtained by displacing the electron emitters or bydisplacing both the electron emitters and the storage medium. Moreover,although a particular micromover 112 is shown and described herein, itwill be appreciated by persons having ordinary skill in the art thatalternative moving means could be employed to obtain such relativemovement.

Alignment of an emitted beam and storage area can be further facilitatedwith deflectors (not shown). By way of example, the electron beams canbe rastered over the surface of the storage medium 110 by eitherelectrostatically or electromagnetically deflecting them, as through useof electrostatic and/or electromagnetic deflectors positioned adjacentthe emitters 108. Many different approaches to deflect electron beamscan be found in literature on scanning electron microscopy (SEM).

The electron emitters 108 are responsible for reading and writinginformation on the storage areas of the storage medium with the electronbeams they produce. Therefore, the electron emitters 108 preferablyproduce electron beams that are narrow enough to achieve the desired bitdensity for the storage medium 110, and that provide the different powerdensities needed for reading from and writing to the medium. Particularexample embodiments for the electron emitters 108 are provided later inthis disclosure.

As indicated in FIGS. 1 and 2, the data storage device 100 can furtherinclude one or more supports 120 that support the storage medium 110 inplace within the interior space 104. When provided, the supports 120typically comprise thin-walled microfabricated beams that flex when thestorage medium 110 is displaced in the X and/or Y directions. As isfurther indicated in FIGS. 1 and 2, the supports 120 can each beconnected to the walls 106 of the casing 102.

In a preferred embodiment, the electron emitters 108 are containedwithin a two-dimensional array comprising a plurality of emitters. Byway of example, an array of 100×100 electron emitters 108 can beprovided with an emitter pitch of approximately 5 to 100 micrometers inboth the X and Y directions. As discussed above, each emitter 108typically is used to access a plurality of storage areas of the storagemedium 110. FIG. 4 provides a schematic representation of thisrelationship. In particular, this figure illustrates a single electronemitter 108 positioned above a plurality of storage areas 400 of thestorage medium 110. As indicated in FIG. 4, the storage areas 400, likethe electron emitters 108, are contained in a two-dimensional array. Inparticular, the storage areas 400 are arranged in separate rows 402 andcolumns 404 on the surface of the storage medium 110. In a preferred anembodiment, each emitter 108 is only responsible for a portion of theentire length of predetermined numbers of rows 402. Accordingly, eachemitter 108 normally can access a matrix of storage areas 400 ofparticular rows 402 and columns 404. Preferably, each row 402 that isaccessed by a single electron emitter 108 is connected to a singleexternal circuit.

To address a storage area 400, the micromover 112 is activated todisplace the storage medium 110 (and/or electron emitters 108) to alignthe storage area with a particular electron emitter. Typically, eachemitter 108 can access tens of thousands to hundreds of millions ofstorage areas 400 in this manner. The storage medium 110 can have aperiodicity of approximately 1 to 100 nanometers between any two storageareas 400, and the range of the micromover 112 can be approximately5-100 micrometers. As will be appreciated by persons having ordinaryskill in the art, each of the electron emitters 108 can be addressedsimultaneously or in a multiplexed manner. A parallel accessing schemecan be used to significantly increase the data rate of the storagedevice 100.

Writing with the data storage device 100 is accomplished by temporarilyincreasing the power density of an electron beam produced by an electronemitter 108 to modify the surface state of a storage area 400 of thestorage medium 110. For instance, the modified state can represent a “1”bit, while the unmodified state can represent a “0” bit. Moreover, thestorage areas can be modified to different degrees to represent morethan two bits, if desired. In a preferred embodiment, the storage medium110 is constructed of a material whose structural state can be changedfrom crystalline to amorphous by electron beams. An example material isgermanium telluride (GeTe) and ternary alloys based on GeTe. To changefrom the amorphous to the crystalline state, the beam power density canbe increased and then slowly decreased. This increase/decrease heats theamorphous area and then slowly cools it so that the area has time toanneal into its crystalline state. To change from the crystalline toamorphous state, the beam power density is increased to a high level andthen rapidly reduced. Although temporary modification of the storagemedium 110 is described herein, it will be understood that permanentmodification is possible where write-once-read-many (WORM) functionalityis desired.

Reading is accomplished by observing the effect of the electron beam onthe storage area 400, or the effect of the storage area on the electronbeam. During reading, the power density of the electron beam is kept lowenough so that no further writing occurs. In a first reading approach,reading is accomplished by collecting the secondary and/or backscatteredelectrons when an electron beam with a relatively low (i.e., lower thanthat needed to write) power density is applied to the storage medium110. In that the amorphous state has a different secondary electronemission coefficient (SEEC) and backscattered electron coefficient (BEC)than the crystalline state, a different number of secondary andbackscattered electrons are emitted from a storage area 400 whenbombarded with a read electron beam. By measuring the number ofsecondary and backscattered electrons, the state of the storage area 106can be determined.

FIG. 5 illustrates example apparatus for reading according to the firstreading approach. More particularly, FIG. 5 schematically illustrateselectron emitters 108 reading from storage areas 500 and 502 of thestorage medium 110. In this figure, the state of storage area 500 hasbeen modified, while the state of storage area 502 has not. When a beam504 of electrons bombard the storage areas 500, 502 both the secondaryelectrons and backscattered electrons are collected by electroncollectors 506. As will be appreciated by persons having ordinary skillin the art, modified storage area 500 will produce a different number ofsecondary electrons and backscattered electrons as compared tounmodified storage area 502. The number may be greater or lesserdepending upon the type of material and the type of modification made.By monitoring the magnitude of the signal current collected by theelectron collectors 506, the state of and, in turn, the bit stored inthe storage areas 500 and 502 can be identified.

In another reading approach, a diode structure is used to determine thestate of the storage areas 400. According to this approach, the storagemedium 110 is configured as a diode which can, for example, comprise ap-n junction, a schottky barrier, or substantially any other type ofelectronic valve. FIG. 6 illustrates an example configuration of such astorage medium 110. It will be understood that alternative diodearrangements (such as those shown in U.S. Pat. No. 5,557,596) arefeasible. As indicated in this figure, the storage medium 110 isarranged as a diode having two layers 600 and 602. By way of example,one of the layers is p type and the other is n type. The storage medium110 is connected to an external circuit 604 that reverse-biases thestorage medium. With this arrangement, bits are stored by locallymodifying the storage medium 110 in such a way that collectionefficiency for minority carriers generated by a modified region 608 isdifferent from that of an unmodified region 606. The collectionefficiency for minority carriers can be defined as the fraction ofminority carriers generated by the instant electrons that are sweptacross a diode junction 610 of the storage medium 110 when the medium isbiased by the external circuit 604 to cause a signal current 612 to flowthrough the external circuit.

In use, the electron emitters 108 emit narrow beams 614 of electronsonto the surface of the storage medium 110 that excite electron-holepairs near the surface of the medium. Because the medium 110 isreverse-biased by the external circuit 604, the minority carriers thatare generated by the incident electrons are swept toward the diodejunction 610. Electrons that reach the junction 610 are then sweptacross the junction. Accordingly, minority carriers that do notrecombine with majority carriers before reaching the junction 610 areswept across the junction, causing a current flow in the externalcircuit 604.

As described above, writing is accomplished by increasing the powerdensity of electron beams enough to locally alter the physicalproperties of the storage medium 110. Where the medium 110 is configuredas that shown in FIG. 6, this alteration affects the number of minoritycarriers swept across the junction 610 when the same area is radiatedwith a lower power density read electron beam. For instance, therecombination rate in a written (i.e., modified) area 608 could beincreased relative to an unwritten (i.e., unmodified) area 606 so thatthe minority carriers generated in the written area have an increasedprobability of recombining with minority carriers before they have achance to reach and cross the junction 610. Hence, a smaller currentflows in the external circuit 604 when the read electron beam isincident upon a written area 608 than when it is incident upon anunwritten area 606. Conversely, it is also possible to start with adiode structure having a high recombination rate and to write bits bylocally reducing the recombination rate. The magnitude of the currentresulting from the minority carriers depends upon the state ofparticular storage area, and the current continues the output signal 612to indicate the bit stored.

As identified above, various hurdles exist to the use of Spindt (i.e.,tip) electron emitters. Accordingly, alternative emitter configurationsare contemplated. Generally speaking, these alternative electronemitters comprise ballistic or quasi-ballistic electron emitters. Moreparticularly, the electron emitters are configured as flat emitters.FIG. 7 illustrates a first example flat electron emitter 700 that can beused in the data storage device 100 to bombard a target 702 (e.g.,storage medium 110). As indicated in this figure, the emitter 700includes an n++ semiconductor substrate 704 that, for example, can bemade of silicon. Typically, the thickness of the substrate depends uponthe size of the wafer used to form the substrate. By way of example, thesubstrate 704 can be approximately 400 to 1000 micrometers thick. Thesubstrate 704 is fabricated such that it includes a volcano-like,funnel-like, or nozzle-like active region 706. Stated in other words,the active region 706 generally has a wide base that quickly narrowsinto a neck 708.

The active region 706 is surrounded by an isolation region 710 thatlimits the geometry of the active region 706 to limit the area fromwhich the active region can emit electrons. By way of example, theisolation region 710 comprises silicon dioxide that is formed through anoxidation process (e.g., wet or dry oxidation). In addition to limitingthe geometry of the active region 706, the isolation region 710 isolatesthe active region 706 from neighboring active regions (not shown).However, it will be understood that bases of the active regions 706 ofcontiguous electron emitters 700 can be connected together.

Formed on the substrate 704 is a semiconductor layer 712. By way ofexample, the semiconductor layer 712 is made of polysilicon or siliconcarbide (SiC) and has a thickness of approximately 0.01 to 2micrometers. In a preferred arrangement, the semiconductor layer 712includes a planar outer surface 714 and a porous region 716. Asindicated in FIG. 7, the porous region 716 is limited in extent suchthat it is aligned with the neck 708 of the active region 706. Limitingthe porous region in this manner allows for higher current densities dueto increased thermal energy dissipation. The porous region 716terminates at the outer surface 714 to define an emission surface 718.In that the surface 714 preferably is planar, the emission surface 718likewise preferably is planar. This configuration permits betterfocusing of electron beams emitted from the emitter 700. By way ofexample, the area of the emission surface 718 can be limited to lessthan approximately 10% of the total area of the outer surface 714 of thesemiconductor layer 712. Most preferably, the area of the emissionsurface 718 is limited to less than approximately 1% of the total areaof the surface 714.

The electron emitter 700 further includes an emission electrode 720formed on the semiconductor layer 712 that is used to supply voltage tothe semiconductor layer 712. The emission electrode 720 typically iscomposed of a highly electrically conductive material such as chromiumand can have a thickness of approximately 0.1 to 1 micrometer. Inaddition to the emission electrode 720, the emitter 700 includes aconductive layer 722 that covers the emission electrode 720 and aportion of the outer surface 714 of the semiconductor layer 712,including the emission surface 718. This layer 722 is preferably thinand can, for instance, have a thickness of approximately 10 nanometersor less. The conductive layer 722 provides an electrical contact overthe emission surface 718 and allows an electric field to be applied overthe emission surface 718. Preferably, the conductive layer 722 comprisesan alloy that does not form an insulating oxide or nitride on itssurface to avoid the creation of tunnel barriers that would negativelyeffect the efficiency of the electron emitter 700.

By way of example, the conductive layer 722 can be made of a thin metalor conductive material such as gold, carbon (e.g., graphite,electrically conductive diamond, or combinations thereof), platinum,iridium, rhodium, conductive boron nitride, or other conductors orsemiconductors. Generally speaking, materials having atomic numberssubstantially below that of gold may also be used for the conductivelayer 722 in that such materials do not scatter electrons (which lowersemission efficiency) to the extent that materials having higher atomicnumbers do. As a low atomic number element, carbon exhibits very lowelectron scattering probability. The conductive layer 722 can be porousor semi-dense such that all conductive areas are electrically connected.For example, the conductive layer 722 can include electricallyinterconnected conductive islands, a mesh of interconnected filaments,or a combinations thereof. In an alternative embodiment, the conductivelayer 722 can comprise multiple thin layers 800 of metal, as shown inthe detail view of FIG. 8.

The electron emitter 700 can further include a back contact 724 that isformed on the substrate 704 on a side opposite that on which thesemiconductor layer 712 is formed. When provided, the back contact 724establishes an equipotential surface for internal fields in thesemiconductor substrate 704 and the porous region 716. It is to beunderstood that the back contact 724 can be eliminated if the substrate704 is highly doped, in which case a contact can be made to thesubstrate via a front contact through known means.

During operation, different potentials are applied (e.g., with on oroff-chip drivers) to the substrate 704, the emission electrode 712, andthe back contact 724. The resulting emission electrode voltage causeselectrons to be injected from the active region 706 of the substrate 704into the porous region 716 of the semiconductor layer 712 and be emittedfrom the emission surface 718 and through the conductive layer 722. Thisemission results in an electron beam 726 that impinges the target 702.

As will be appreciated by persons having ordinary skill in the art,focusing means may be needed to focus the beam 726 on the target 702.One example of such focusing means are illustrated in FIG. 9 whichillustrates a second example flat electron emitter 900. As indicated inthis figure, the emitter 900 is similar in several ways to the emitter700 shown in FIG. 7. Accordingly, the emitter 910 comprises a substrate704 including an active region 706 and an isolation region 710, asemiconductor layer 712 including a porous region 716, an emissionelectrode 720, a conductive layer 722, and a back contact 724. Inaddition, the electron emitter 900 includes a focusing structure 902that is used to focus the electron beams emitted from the emitter 900.

As shown in FIG. 9, the focusing structure 902 comprises an insulatinglayer 904, a lens electrode 906, and a second conductive layer 908. Theinsulating layer 904 isolates the emission electrode 720 from the lenselectrode 906. Like the conductive layer 722, the conductive layer 908provides a contact over the lens electrode 906 such that an electricfield can be applied thereto. As indicated in FIG. 9, the lens electrode906 and the conductive layer 908 are formed so as to define an aperture910 through which electron beams can pass. In use, a potential isapplied to the lens electrode 906. The electric field resulting from thelens electrode voltage at the aperture 910 causes the emitted electronsto be focused. Typically, this focus can be adjusted by varying thepotential applied to the lens electrode 906. The electron beam can befocused to a very small spot size, e.g., less than 1 nanometer indiameter, on the target (not shown). Although a particular focusingarrangement has been shown and described, it will be appreciated bypersons having ordinary skill in the art that many different focusingarrangements are possible and that others may even be more preferable.

FIG. 10 illustrates a third example flat electron emitter 1000 that canbe used in the data storage device 100. The electron emitter 1000includes a n++ semiconductor substrate 1002 and a semiconductor layer1004 that is formed on the substrate. By way of example, the substrate1002 can comprise silicon and the layer can comprise polysilicon. Inaddition, the emitter 1000 includes an insulating layer 1006, apatterning mask 1008, and a conductive layer 1010. The patterning mask1008 is deposited on the semiconductor layer 1004 and the insulatinglayer 1006. In similar manner, the conductive layer 1010 is deposited onthe patterning mask 1008 and the semiconductor layer 1004. Thesemiconductor layer 1004 includes a porous region 1012. An opening 1014in the patterning mask 1008 defines an emission area 1016 of the emitter1000.

Electron emission can be achieved with emitter structures distinct fromthose described above. For example, the electron source may be adaptedto emit electrons from the surfaces of metal-insulator-metal (MINI) andmetal-insulator-oxide (MIS) structures at or below room temperature.This type of electron emission is described in Wade & J Briggs, “Lownoise Beams from Tunnel Cathodes,” Journal of Applied Physics 33, No. 3,pp. 836-840, 1962; Julius Cohen, “Tunnel Emission into Vacuum,” AppliedPhysics Letters 1, No 3, pp. 61-62, 1962; and Yokoo, et al., “Emissioncharacteristics of metaloxide-semiconductor electron tunneling cathode,”Journal of Vacuum Science and Technology, pp. 429-432, 1993. Electronsfrom MIM and MIS structures are emitted into the vacuum with smalldivergence angles as described in R. Hrach, Thin Solid Films 15, p. 15,1973. Small divergence angles allow the emitted electrons to be focusedinto small diameter electron beams.

FIG. 11 shows a flat electron emitter 1100 that includes a MIM-basedelectron emission structure. As indicated in this figure, the emitter1100 includes a substrate 1102 including an active region 1104 and anisolation region 1106, an insulator layer 1108, an emission electrode1110, a conductive layer 1112, and a back contact 1114. Included in theactive region 1104 of the substrate 1102 adjacent the insulator layer1108 is a thin metal layer 1116. Therefore, a metal-insulator-metalarrangement is obtained by the conductive layer 1112, the insulatorlayer 1108, and the metal layer 1116. Although a particular MIMarrangement is shown and described, it will be appreciated by personshaving ordinary skill in the art that alternative arrangements arefeasible.

FIG. 12 shows a flat electron emitter 1200 that includes a MIS-basedelectron emission structure. As indicated in this figure, the emitter1200 includes a silicon substrate 1202 including an active region 1204and an isolation region 1206, an insulator layer 1208, an emissionelectrode 1210, a conductive layer 1212, and a back contact 1214. Themetal-insulator-silicon arrangement is obtained by the conductive layer1212, the insulator layer 1208, and the substrate 1202. Although aparticular MIS arrangement is shown and described, it will beappreciated by persons having ordinary skill in the art that alternativearrangements are feasible.

While particular embodiments of the invention have been disclosed indetail in the foregoing description and drawings for purposes ofexample, it will be understood by those skilled in the art thatvariations and modifications thereof can be made without departing fromthe scope of the invention as set forth in the following claims.

What is claimed is:
 1. A data storage device, comprising: a plurality ofelectron emitters adapted to emit electron beams, each electron emitterhaving a planar emission surface; and a storage medium in proximity tothe electron emitters, the storage medium having a plurality of storageareas that are capable of at least two distinct states that representdata, the state of each storage area being changeable in response tobombardment by an electron beam emitted by an electron emitter; whereindata is written to the device by changing the state of the storage areasand data is read by the device by observing phenomena relevant to thestorage areas.
 2. The device of claim 1, further comprising electroncollectors positioned so as to receive secondary and backscatteredelectrons produced by the storage areas in response to incident electronbeams.
 3. The device of claim 1, wherein the storage medium is a diodeand the storage areas produce minority carriers in response to incidentelectron beams.
 4. The device of claim 1, wherein the electron emitterseach comprise a semiconductor layer having an outer surface that formsthe emission surface.
 5. The device of claim 4, wherein the emissionsurface occupies an area that comprises a small fraction of the totalarea of the outer surface.
 6. The device of claim 5, wherein theemission surface occupies an area that represents approximately lessthan 10% of the total area of the outer surface.
 7. The device of claim4, wherein the semiconductor layer includes a porous region that extendsthrough the semiconductor layer to the emission surface.
 8. The deviceof claim 7, further comprising a substrate on which the semiconductorlayer is formed, the substrate having an active region that narrows to aneck that has a cross-sectional area that approximates a cross-sectionalarea of the porous region.
 9. The device of claim 8, wherein the activeregion has a funnel-like shape.
 10. The device of claim 1, furthercomprising a conductive layer formed on the planar emission surface. 11.The device of claim 10, wherein the conductive layer has a thickness ofapproximately 10 nanometers or less.
 12. The device of claim 10, whereinthe conductive layer comprises multiple metal layers.
 13. The device ofclaim 1, further comprising focusing structures that focus electronbeams emitted from the electron emitters.
 14. The device of claim 13,wherein the focusing structures define apertures through which emittedelectron beams pass.
 15. The device of claim 14, wherein the focusingstructures each include a lens electrode that defines the aperture. 16.The device of claim 1, further including patterning masks that definethe planar emission surfaces.
 17. The device of claim 1, wherein theelectron emitters are metal-insulator-metal (MIM) electron emissionstructures.
 18. The device of claim 1, wherein the electron emitters aremetal-insulator-silicon (MIS) electron emission structures.
 19. A datastorage device, comprising: means for emitting electron beams, the meansincluding planar emission surfaces; and means for storing data capableof at least two distinct states that represent data, the state beingchangeable in response to bombardment by an electron beam emitted by themeans for emitting electron beams; wherein data is written to the deviceby changing the state of the means for storing data and data is read bythe device by observing phenomena relevant to the means for storingdata.
 20. The device of claim 19, further comprising means forcollecting secondary and backscattered electrons produced by the meansfor storing data in response to incident electron beams.
 21. The deviceof claim 19, wherein the means for storing data comprise a diode thatproduces minority carriers in response to incident electron beams. 22.The device of claim 19, further comprising means for focusing electronbeams emitted from the means for emitting electron beams.
 23. A methodfor storing data, comprising the steps of: emitting an electron beamfrom an electron emitter including a planar emission surface; directingthe electron beam toward a storage medium comprising a plurality ofstorage areas; and bombarding one of the storage areas with electronswith the electron beam so as to change a state of the storage area. 24.The method of claim 23, wherein the storage area is changed from acrystalline state to an amorphous state.
 25. The method of claim 23,wherein the storage area is changed from an amorphous state to acrystalline state.
 26. The method of claim 23, further comprising thestep of collecting secondary and backscattered electrons produced by thestorage area in response to an incident electron beam to determine thestate of the storage area.
 27. The method of claim 23, furthercomprising the step of observing the effect on minority carriersproduced by the storage area in response to an incident electron beam todetermine the state of the storage area.
 28. A data storage device,comprising: a plurality of electron emitters adapted to emit electronbeams, the electron emitters each including a substrate having an activeregion and a semiconductor layer formed on the substrate, thesemiconductor layer including a planar outer surface that forms a planaremission surface; and a storage medium in proximity to the electronemitter, the storage medium having a plurality of storage areas that arecapable of at least two distinct states that represent data, the stateof the storage areas being changeable in response to bombardment byelectron beams emitted by the electron emitters; wherein data is writtento the device by changing the state of the storage areas and data isread by the device by observing phenomena relevant to the storage areas.29. The device of claim 28, wherein the emission surface occupies anarea that represents approximately less than 10% of a total area of theouter surface.
 30. The device of claim 28, wherein the semiconductorlayer includes a porous region that extends through the semiconductorlayer to the emission surface.
 31. The device of claim 28, wherein thesubstrate has an active region that narrows to a neck that has across-sectional area that approximates a cross-sectional area of theporous region.
 32. The device of claim 28, further comprising aconductive layer formed on the planar emission surface.
 33. The deviceof claim 32, wherein the conductive layer has a thickness ofapproximately 10 nanometers or less.
 34. The device of claim 32, whereinthe conductive layer comprises multiple metal layers.
 35. The device ofclaim 28, further comprising focusing structures that focus electronbeams emitted from the electron emitters.